1. Field of the Invention
2. Description of the Related Art
The present invention relates to polymer compositions and in particular to compositions that are suitable for the preparation of articles, especially thin walled articles, by moulding, thermoforming and extrusion. The invention also concerns processes for preparing such articles and to the resulting articles per se.
Propylene polymers, especially heterophasic propylene polymers, have excellent mechanical properties, therefore polypropylene is the polymer of choice for the production of many types of article (e.g. containers, lids, pails, crates, boxes, buckets, etc.) To provide polypropylene articles having, for example, high impact resistance and rigidity, polypropylene is often used in conjunction with a nucleating agent that serves to increase its crystallinity by increasing the rate of crystallisation. Various different nucleating agents have been used including aromatic carboxylic acids, aromatic metal phosphates and sorbitol derivatives.
Another agent that is known to have a nucleating effect is a polymer of vinyl cyclohexane. EP-A-0151883, for example, discloses a propylene polymer composition comprising crystalline polypropylene and a polymer of a vinyl cycloalkane and teaches that, unlike other nucleating agents, the use of polymers of vinyl cycloalkanes improves both the crystallinity and the transparency of final products. The examples of EP-A-0151883 show that sheets comprising a crystalline polypropylene and a polymer of a vinyl cycloalkane have a low light scattering index, low haze and high gloss compared to sheets prepared solely from polypropylene.
U.S. Pat. Nos. 6,503,993 and 6,437,063 also disclose propylene polymer compositions comprising a polymer of a vinyl cycloalkane. U.S. Pat. No. 6,503,993, for example, discloses a polypropylene nucleated with 0.001 to 1% wt of a polymerised vinyl compound and teaches that it may be blended with 5-30% wt rubbery copolymer or other polymers. The resulting compositions are said to exhibit higher shrinkage than conventional polypropylene containing talc but within the tolerance limits of many manufacturers.
WO 99/24501 discloses, however, that whilst the crystallisation properties of polymers comprising polymers of vinyl cycloalkanes are excellent, that compositions comprising such polymers exhibit high levels of shrinkage. More specifically it has been found that these compositions have a tendency, after moulding, thermoforming and/or extrusion, to undergo significantly increased amounts of shrinkage during cooling compared to their counterparts that do not contain a polymer of vinyl cycloalkane.
Moreover the extent of shrinkage in polymer compositions comprising polymers of vinyl cycloalkane has been found to vary with, amongst other factors, the size of the resulting article. Such variable shrinkage is highly undesirable since it precludes the use of such compositions for the production of articles where their final dimensions are critical. This is the case, for example, with containers (e.g. pails) and lids, caps or closures that are required to fit together. Manufacturers of such articles therefore tend to have low shrinkage “tolerances” (i.e. the level of shrinkage should be low so that any differences are minimised).
To try to control shrinkage of polypropylene compositions comprising polymers of vinyl cycloalkane. WO 99/24501 teaches the use of talc. More specifically WO 99/24501 describes polypropylene compositions comprising polypropylene comprising a polymer of vinyl cycloalkane and 0.1 to 10% talc. It is also shown in WO 99/24501 that articles formed from these compositions have improved stiffness, a higher tensile modulus and an increased Heat Deflection Temperature than those compositions wherein the polymeric nucleating agent is absent.
The compositions described in WO 99/24501 are prepared by processes wherein a propylene polymer comprising a polymer of vinyl cycloalkane is first prepared and then mixed with talc. For example, a vinyl cycloalkane compound may first be polymerised in the presence of a propylene polymerisation catalyst to prepare a modified catalyst that is used in the polymerisation of propylene. Alternatively propylene polymer and a polymer of vinyl cycloalkane can be prepared separately and mixed by, for example, melt kneading. In both cases, talc is added in the last stage of the process.
Whilst the processes described in WO 99/24501 provide propylene having desirable properties, they require the addition of talc as well as fairly high amounts (e.g. up to 10,000 ppm wt) of the polymer of a vinyl cycloalkane. The polymer of a vinyl cycloalkane is, however, relatively expensive.